This invention is generally directed to imaging members and more specifically the present invention is directed to layered photoconductive imaging members. In one embodiment of the present invention, the imaging members are comprised of a supporting substrate, a photogenerating layer and a charge transport layer comprised of any one of the fullerenes, such as buckminsterfullerene, giant fullerenes or mixtures thereof optionally dispersed in a resin binder. There are also provided in accordance with the present invention imaging members comprised of a supporting substrate, a photogenerating layer comprised of any one of the fullernes, such as buckminsterfullerene, giant fullerenes or mixtures thereof optionally dispersed in a resin binder and a charge transport layer. The imaging members of the present invention are useful in electrostatographic imaging systems, especially xerographic imaging and printing processes.
Advantages associated with the present invention in embodiments thereof, and especially when the charge transport is comprised of a fullerene include, for example, the economical noncomplex preparation of photoconductive imaging members in the form of vacuum sublimed layers without any decomposition or degradation of the fullerene molecular structure since it is believed that the fullerenes possess both physical, photochemical and photophysical stability. Also, the very high symmetry of the C.sub.60 molecule, for example, as compared with the planar or one-dimensional structure of many known organic sensitizing or transport molecules diminishes the influence of steric or orientational effects on molecular wave function overlap in the resulting imaging member. This offers additional latitude in achieving and maintaining the optimum sensitizing and charge transport efficiency. For imaging applications, photosensitivites (number of carriers produced per absorbed photon) should preferably be unity, although values between 0.1 and 1.0 can be useful; for electrophotographic purposes the charge transport efficiency should be such that the distance a carrier can move per unit field, the so called .mu..tau. product should be .gtoreq.10.sup.-6 cm.sup.2 /volts. Additionally, the cage structure of the fullerenes offers considerable scope for the achievement and production of designer molecules to achieve these desirable ranges of electronic transport and spectral sensitivity and solubility in solvents and matrices up to the desired range of 20 to 40 percent by weight through the chemical attachment of appropriate molecular chromophores, or electronically active molecules, such as aryl amines, to the basic fullerene molecule.
Molecular fullerenes have been described as entirely closed, hollow spheroidal shells of carbon atoms containing 32 to 1,000 or more carbon atoms in each sphere, reference Smalley, R. E. "Supersonic Carbon Cluster Beams in Atomic and Molecular Clusters", Bernstein, E. R.; and Physical and Theoretical Chemistry, Vol. 68, Elsevier Science: New York, 1990; pages 1 to 68, the disclosures of which are totally incorporated herein by reference. The prototypical fullerene, C.sub.60, has been referred to as buckminsterfullerene and has the molecular geometry of a truncated icosahedron, thus the C.sub.60 molecules resemble a molecular sized soccer ball, reference Time Magazine, May 6, 1991, page 66, and Science, vol. 252, Apr. 2, 1991, page 646, the disclosures of which are totally incorporated herein by reference. Molecules of C.sub.60, C.sub.70 and of other fullerenes have also been referred to as buckyballs. Buckminsterfullerenes are usually comprised of C.sub.60 molecules contaminated with small amounts of C.sub.70 and possibly C.sub.84 molecules or even smaller amounts of higher molecular weight fullerene molecules. The preparation of buckminsterfullerene and of other fullerenes from the contact arc vaporization of graphite and a number of the buckminsterfullerene characteristics, such as solubility, crystallinity, color and the like, have been described in Kratschmer, W., Lamb, L. D., Fostiropoulos, K., Huffman, D. R. Nature, 1990, Vol. 347, pages 354 to 358 and in Chemical and Engineering News, Oct. 29, 1990, pages 22 to 25, the disclosures of which are totally incorporated herein by reference. The fullerenes are available from Texas Fullerenes Corporation, 2415 Shakespeare Suite 5, Houston, Tex. 77030-1038, Materials and Electrochemical Research (MER) Corporation, 7960 South Kolb Road, Tucson, Ariz. 85706, and Research Materials, Inc., 1667 Cole Boulevard, Golden, Colo. 80401, and are believed to be comprised of mainly C.sub.60 and smaller amounts of C.sub.70 and C.sub.84 carbon molecules, and possible small amounts of other higher molecular weight fullerenes. It is believed that these new forms of carbon possess a number of advantages for electrophotographic applications, including, for example, their solubility in organic solvents. The other known carbon forms, diamond and graphite and derivatives thereof, are not considered to be soluble in such solvents. Solubility in organic solvents enables improved processing and the economical preparation of compositions wherein the optical density is considered low since the fullerenes are of different colors and are of substantially lower optical density than ordinary carbon black. Allotropic forms of carbon comprised of spherical assemblies of carbon atoms C.sub.n with, for example, n being the number 60, 70, 84, and the like are considered fullerenes and can be formed as powders by the evaporation of graphite in inert noble gas atmospheres with arcs or lasers, and these fullerenes are available from the sources mentioned herein. The color of the allotrope can depend on the value of n, for example when n is equal to 70 the color is orange, when n is equal to 84 the color is purple magenta, and when n is equal to 60 the color is yellow.
There was submitted in July 1991 to Nature, 1137 National Press Building Washington, D.C. 20045 for review by a referee and for later possible publication, the disclosure of which is totally incorporated herein by reference, a letter of which the following is a summary thereof. Reports exist on the electrical properties of C.sub.60 and C.sub.70 films doped with alkali metals, including the observation of superconductivity. Undoped films are insulators and yet have significant visible absorption. We report photoeffects in the visible and near-infrared spectrum in sublimed films of C.sub.60/70. The peak photoefficiency (photocarrier per absorbed photon) is .about.10.sup.-4. Since for wavelengths &gt;7,000 .ANG., absorption in C.sub.60/70 films is very weak, carrier photoinjection from the electrodes into the conduction states of C.sub.60/70 must be considered. Evidence, based on the energy level structure of C.sub.60 films is discussed which suggests this may not be the source of the photocurrents. Alternatively, the observed threshold may be the photoconductive edge associated with the reported weak absorption due to forbidden transitions in the lowest direct bandgap, which recent calculations place at 1.5 electron volts. The samples were deposited on glass slides, partially precoated with evaporated aluminum electrodes, by vacuum sublimation of C.sub.60/70 prepared as previously described. The ratio of C.sub.70 to C.sub.60 in the source material was .about.0.1. We have assumed that the controlling molecule in the films is C.sub.60, although C.sub.70 may also play a significant role. Sandwich cells were completed by evaporating semitransparent, top electrodes (area 0.3 cm.sup.2) for which gold or aluminum were used. The sandwich cell geometry, in which the current flow is perpendicular to the plane of the film, minimizes the possible complications of more conductive surface layers which, if present, can dominate surface cell measurements. In addition, the films were of sufficient thickness, 1.5 .mu.m, to avoid shorting due to discontinuities or pinholes and yet allowed sufficiently high fields to be applied to increase the possibility of detecting photoconductivity. Dark conductivity measurements made on these samples have established that the room temperature dark conductivity is .about.10.sup.-14 (.OMEGA.cm).sup.-1, and that the dark current versus voltage curves are linear for applied voltages less than 2 volts. For the photoeffect measurements, a xenon discharge lamp in combination with isolation filters was employed to achieve the requisite higher light intensities than achievable with a monochromator.
Layered photoconductive imaging members with charge generating and charge transport layers are known, reference for example U.S. Pat. No. 4,265,990, the disclosure of which is totally incorporated herein by reference. Photoreceptor materials comprising inorganic or organic materials wherein the charge generating and charge transport functions are performed by discrete contiguous layers are known. Additionally, layered photoreceptor members are disclosed in the prior art, including photoreceptors having an overcoat layer of an electrically insulating polymeric material. Photoresponsive materials containing a hole injecting layer overcoated with a hole transport layer, followed by an overcoating of a photogenerating layer, and a top coating of an insulating organic resin, are disclosed in U.S. Pat. No. 4,251,612, the disclosure of which is totally incorporated herein by reference. Examples of photogenerating layers disclosed in these patents include trigonal selenium and phthalocyanines, while examples of transport layers include certain aryl diamines as illustrated therein.
In addition, U.S. Pat. No. 3,041,167 discloses an overcoated imaging member containing a conductive substrate, a photoconductive layer, and an overcoating layer of an electrically insulating polymeric material. This member can be employed in electrophotographic imaging processes by initially charging the member with an electrostatic charge of a first polarity, followed by exposing it to form an electrostatic latent image that can subsequently be developed to form a visible image. Composite electrophotographic photosensitive materials containing various azo compounds are disclosed in U.S. Pat. No. 4,618,672, wherein bisazo compounds particularly suitable for use in the charge generating layer of a layered electrophotographic photoconductor are illustrated. Similarly, an article by M. Hashimoto entitled "Electrophotographic Sensitivity of Fluorenone Bisazo Pigments", Electrophotography, Vol. 25, No. 3 (1986), discloses disazo compounds as charge generating materials in electrophotographic layered photoreceptors. Further, Japanese Patent Kokai No. 54-20736 discloses disazo pigments as constituents in electrophotographic processes. Japanese Patent 58-177955 also discloses many disazo compounds suitable for use in the photosensitive layer of an electrophotographic device.
U.S. Pat. No. 4,713,307, the disclosure of which is hereby totally incorporated by reference, also discloses photoconductive imaging members containing a supporting substrate, certain azo pigments as photogenerating materials, and a hole transport layer that preferably contains an aryl amine compound dispersed in an inactive resinous binder.
U.S. Pat. No. 4,797,337, the disclosure of which is totally incorporated herein by reference, discloses a photoconductive imaging member comprising a supporting substrate, a hole transport layer, and a photogenerating layer comprising specific disazo compounds.
U.S. Pat. No. 4,755,443 discloses a photoreceptor for electrophotography which comprises a charge carrier generating material and charge transport material wherein one charge generating material is a metal phthalocyanine or a metal free phthalocyanine. Other carrier generating substances can be used in combination with the phthalocyanine generator material, including azo pigments, anthraquinone dyes, perylene dyes, polycyclic quinone dyes, and methine stearate pigments.
U.S. Pat. No. 4,424,266 discloses an electrophotographic photosensitive element having a conductive support and a photosensitive layer comprising a carrier generating phase layer containing a carrier generating material selected from the group consisting of perylene dyes, polycyclic quinones, and azo dyes, and a carrier transporting phase layer containing a hydrazone carrier transporting material. The carrier generator materials can be used either singly or in combination.
Illustrated in copending patent application U.S. Ser. No. 709,734, the disclosure of which is totally incorporated herein by reference, are developer compositions and toner compositions comprised of resin particles, and pigment particles comprised of fullerenes, a new third form of carbon also referred to as buckminsterfullerene or buckyballs, other forms of fullerenes illustrated therein, and other known fullerenes. More specifically, the copending patent application discloses toner compositions comprised of resin particles, and pigment particles comprised of fullerenes, a third form of carbon, described as being comprised of 60 atom clusters of carbon arranged at the verticies of a truncated icosahedron and resembling miniature soccer balls. Such a structure resembles the geodesic domes designed by R. Buckminister Fuller, Jr., the namesake of these molecular structures. In one embodiment of the copending application, there are provided toner compositions comprised of resin particles, pigment particles, and fullerenes as charge additives. Also, in another embodiment of the copending application there are provided colored toner compositions comprised of known toner resin particles, fullerene pigment particles, and pigment particles comprised of cyan, magenta, yellow, red, green, blue, brown, or mixtures thereof.
Reference to fullerenes includes all forms of the fullerenes illustrated herein, other known fullerenes, mixtures thereof in embodiments, and the like.